In order for a multi-cylinder internal combustion engine to have well controlled engine discharge responsiveness to acceleration or deceleration, the intake system needs to be able to vary the capacity or volume of an intake passage by a throttle valve in accordance with engine operating conditions. For this purpose, some of this kind of intake systems are provided with throttle valves in separate intake passages near intake ports of the cylinders, respectively, in addition to a main throttle valve.
For another purpose, intake systems for use with multi-cylinder internal combustion engines equipped in particular with superchargers are provided with open-and-shut, or switching, valves in the separate intake passages near the intake ports of the cylinders, respectively. This switching valve serves to shut intake air into the cylinders under light engine loads and prevent a reflux of combustion gas, as dilution gas, into the intake passage. This is desirable, because in a typical design of an internal combustion engine with a supercharger a valve overlap period is used. That is the intake valve is opened earlier and the exhaust valve is closed late so that for a period of time both intake and exhaust valves are open. The result is to harness or utilize the moving mass of exhaust flow as a sort of vacuum cleaning effect not only to draw out residual combustion gasses in a combustion chamber, but also, thereby to help initiate fuel induction flow. In such an internal combustion engine, the exhaust gas or gasses reflux in excess into the intake passage especially under light engine loads when manifold pressure delivered by the supercharger is lowered due to the longer valve overlap period, so that an inordinate amount of dilution gas is produced and as a result, deteriorates combustion stability. Such an intake system is known from, for instance, Japanese Unexamined Patent Publication No. 64-15439.
When using an intake system, with throttle valves in the separate branch intake passages, of an internal combustion engine not equipped with a supercharger, the capacity or volume of each separate branch passage downstream the throttle valve (which is hereinafter referred to as effective downstream volume), is small because no interactive pressure is produced among the cylinders, and therefore, intake port pressure rises and reaches almost atmospheric pressure immediately before the valve overlap period or the beginning of opening of the intake. Accordingly, the combustion or exhaust gas, being returned as reflux into the separate branch passage during the valve overlap period, decreases under light engine loads, resulting in a decrease in combustion or exhaust gas consumed as dilution gas during a following intake cycle, so as thereby to improve the stability of combustion.
On the other hand, under heavy engine loads when the engine provides a good deal of power, it is desirable to increase the reflux of combustion gas to some extent for the purpose of reducing harmful emissions including oxides of nitrogen (NOx) or dropping pumping loss so as to improve fuel economy. Nevertheless, the smallness of the effective downstream volume of the separate intake passage, due to the provision of the throttle valve in the separate intake passage, causes a lack of the reflux of combustion or exhaust gas into the separate passage under the heavy engine loads.
To provide an increase in dilution gas, it is thought to close the exhaust valve even later so as to return combustion gas that has been drawn out into an exhaust passage, into the combustion chamber during transition from exhaust to intake cycle and thereby to increase the dilution gas. However, this technique causes dilution gas to increase even under light engine loads and as a result, the stability of combustion is adversely effected.
As is apparent from the above discussion, it is difficult for this kind of intake systems to provide a desired amount of dilution gas and a favorable stability of combustion for both light and heavy engine loads.